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Wounded 5" Petzval - Part 1

Started by tomrod, 05/27/2002 03:33PM
Posted 05/27/2002 03:33PM Opening Post
A few years ago I bought a 5" Petzval OTA from a surplus store. It is an interesting design with Clark objective and a two element 5" coma corrector. The last surface of the corrector is 5" in diameter with a 1.2" sagitta. It had a 3.5" x 4.5" ground glass viewing plate at the focal plane. I cut 6" off the end of the tube, added a focuser, installed a dozen baffles and had a nice refractor. The focuser tube extends up to an inch into the sagitta of the last lens.

I noticed that there was a lot of ghost reflections when looking at Jupiter. Looking through the scope I noticed that the coating on one of the lenses was mottled and I thought that was the cause (wrong). I posted a question on Astromart (I think) and was told that Kodak lens cleaner might remove the coating. I tried it and it worked, but I had to rub hard using old cotton underwear.

When I put it together, it didn’t perform nearly as well as it had. Part of the problem was collimation. I did some star tests and was able to get much better results by rotating lenses in their cells. The outer lens of the objective has four large pock marks and there is astigmatism in that area. The star test also has spikes and I thought that I had put scratches in the lens where I removed the coating. The ghosts turned out to be more from the eyepieces I was using than anything else.

There are only 10 kinds of people who understand binary - those that do and those that don't
Posted 05/27/2002 03:36PM #1
The ghosts turn out to be more from the eyepieces than anything else. I have just disassembled the scope for cleaning and inspection. Using a laser from the side and a microscope (7x to 45x) I initially saw what looked like a very dense starfield at 15x. When I lightly wiped the tiny area under the laser using a cotton pad with isopropyl alcohol I would get streaks in the starfield in the direction of the cleaning. It appeared that there was a film that was from the alcohol or not being removed by it.

I washed the lens in mild soapy water, rinsed it, dried it with cotton pads and lightly wiped it with distilled water. I then did a light wipe with acetone. Now I get a sparse starfield, but I do see some scratches. The scratches are light even under 15x magnification. The scratches are not very numerous, much like a time exposure of a meteor shower with most of the field untouched. The scratches and microscopic pits causing the starfield do not change at all with cleaning so they must be in the glass.

I have not inspected the other lenses carefully, but the reflect the laser much like the tested lens before it was cleaned. From what I have seen, I believe my problem has been from dirty optics more than from scratches in the lenses. Are these tiny pits normal?

I plan to clean all the lenses like I did the first one, check them with the laser this evening and re-assemble the scope. I marked the parts so I can get the scope back together the way I had it earlier, but I could use some advise about collimating a four element scope.

There are only 10 kinds of people who understand binary - those that do and those that don't